Here, just create some Assist Trophies or Pokeball Pokemon of your own. I doubt I have to tell anybody this, but, just make sure your Assist Trophies are video-game related.
Here are two of mine:
Series: Super Mario Bros./Yoshi's Island
Effect: Kamek flies around on his broom and will enlarge whoever released him and shrink any opponents.
Series: Paper Mario
Effect: Dimentio changes the stage the players are fighting on and they'll remain on the new stage until he gets summoned again.